The present invention relates generally to an improved injection molding machine and method of using the machine to form parts. More specifically, the present invention relates to a plunger molding machine for reinforced polymer compositions, particularly, polymers loaded with thermally conductive media, such as carbon and aluminum in the form of fibers and flakes.
In the molding industry, it has been well known to injection mold plastics into various articles of commerce. In particular, it has become well known to load such plastics or polymer-based compositions with other media to form a reinforced polymer composition. Reinforcing a polymer composition with other media is done for many different purposes. For example, reinforced polymer may be employed to provide a thermally conductive plastic where the reinforcing media is highly thermally conductive, such as carbon fiber or aluminum flakes. Another example, is where the polymer is loaded with copper fiber to provide an electrically conductive polymer composition. Still further, aluminum flakes may be loaded in the polymer composition to provide a composition with EMI shielding. Also, glass, carbon or other fiber may be employed to add strength and/or stiffness.
In general, the loading of polymer, with a reinforcing media, raises many concerns as to the ability to successfully injection mold such a mixture because of the presence of the additional reinforcing media. For example, the loading of long carbon fiber into a polymer composition raises concerns as to strand and/or filament breakage during the melting and molding process. There is present the competing issues of the concern of thorough mixing of the loaded composition with the concern of excessive breakage of the delicate reinforcing media. Prior art molding machines typically create high turbulence and/or grinding of the polymer for the purposes of mixing the composition. These prior art machines commonly included a torpedo-shaped member or spreader to increase the level of turbulence to improve turbulence. However, such turbulence and grinding under pressure results in greatly reduced reinforcement media length.
However, these known processes are incompatible with the examples above, particularly the thermally conductive composition with carbon fiber, where it is critical that the breakage or damage to the reinforcing media be kept to a minimum to ensure that the desired properties of the composition are maintained. In the above example, if the lengths of the carbon fibers loaded within the polymer composition are ground up into much shorted lengths, it is clear that the overall thermal conductivity of the composition will be degraded as a result.
To address these problems, compression molding has been attempted where there is a manual lay-up of material and the reinforcing media thereon. As can be understood, such manual assembly is expensive and is far too slow for mass production. Thus, compression molding is inadequate and impractical for molding reinforced material and suffers from economic and geometry-related limitations.
In addition to the problems associated with the reduction of the length of reinforcing media, the alignment of such media is also a concern. In the examples above, a highly aligned and oriented loading of reinforcing media along the path of conductivity is preferred to obtain higher performance of the molded composition. For example, a highly oriented array of carbon fiber within a polymer base would yield higher thermal conductivities than a composition that included randomly oriented fibers because the number of transitions from carbon to polymer to carbon within the composition would be greatly reduced. Further, packing densities are higher when the fibers or filaments are well-aligned. The foregoing alignment and breakage problems become even more important where the aspect ratio of the reinforcing media becomes larger and larger.
In view of the foregoing, there is a demand for an improved injection molding machine and method that is well suited for accommodating polymer compositions loaded with reinforcing media having aspect ratios greater than 1:1. There is a demand for a molding machine that is capable of greatly decreasing the amount of breakage of reinforcing media during the molding process. There is also a demand for a molding machine and method of using the machine that can better align reinforcing media along the line of melt flow to provide a better oriented reinforced composition.